Diagram viewer : check that recorded case folder still exists

Please check if the PR fulfills these requirements

[X] The commit message follows our guidelines
[ ] Tests for the changes have been added (for bug fixes / features)
[ ] Docs have been added / updated (for bug fixes / features)

Does this PR already have an issue describing the problem?

What kind of change does this PR introduce?

Bug fix

What is the current behavior?

If the case folder previously recorded in preferences is removed from the file system the open case dialog is not shown.

What is the new behavior (if this is a feature change)? only set the start folder for open case dialog from the value recorded in preferences if it still an existing directory

Does this PR introduce a breaking change or deprecate an API?

[ ] Yes
[ ] No

If yes, please check if the following requirements are fulfilled

[ ] The Breaking Change or Deprecated label has been added
[ ] The migration steps are described in the following section

What changes might users need to make in their application due to this PR? (migration steps)

Other information:

powsybl / powsybl-dev-tools

Diagram viewer : check that recorded case folder still exists #109

Quality Gate passed